In very high-speed communications systems, which transmit signals at a high frequency, the edge gradient of signals which are produced by transmission drivers from integrated circuits have to comply with specific requirements. This is because the edge gradient of a signal influences at least two major performance features of the communications system. The two major performance features are the signal integrity and the receiver delay. The signal integrity is the governing factor for a function of the quotient dI/dt of the signal and which is itself influenced by parasitic inductances that occur in integrated circuit chips. The receiver delay is influenced by the trigger time for the received signal, which is dependent on the edge gradient. These two factors play a major role in the restriction of the transmission speed between a transmitter and a receiver. It is therefore necessary to stabilize the edge gradient of transmitted signals, in particular of high-power signals which are produced from integrated circuits, with respect to process, voltage and temperature variations and, furthermore, to scale the edge gradient on the basis of the transmission frequency (higher frequencies require proportionally steeper edges).
In many such communications systems, the edge gradient of transmission signals is controlled by two typical methods: according to one method, preamplifier nodes are loaded with resistance/capacitance elements in order in this way to limit the rate at which the output driver amplifiers are switched on and off via their gate. A second method is to in each case switch on some of a number of series-connected amplifier stages, such that the speed at which the transmission driver reaches its full driver level is regulated or controlled. The first method mentioned is based on fixed RC time constants for the preamplifier, and is thus subject to the problem that these time constants vary with the process parameters, with the operating voltage and with the temperature. The second method likewise uses RC time constants for time control between the stages, or else the stage/gate delay, neither of which is stable with respect to process parameter, voltage and temperature fluctuations.
A major disadvantage of the known methods is thus that they cannot keep the edge gradient of transmission signals constant with respect to fluctuations in the process parameters, e.g., the operating voltages and the temperature, nor do they offer any means whatsoever for dynamic scaling of the edge gradient as a function of changing frequencies.